The costs involved in the manufacture of inexpensive solar cells based on crystalline silicon needs to be considerably reduced in relation to the current price level. This presupposes the development of a new technology from the production of a chemically adequate pure silicon from the crystallization process to the molding of the cells.
Moldings of high-purity quartz are commercially obtainable. Since quartz is among the materials which have very little contamination effect on chemically highly pure metal melts, it is advantageously used as crucible material in Czochralski drawing plants in the manufacture of high-purity silicon. By contrast, the use of quartz chill molds for the crystallization of silicon melts involves problems. The coefficient of linear thermal expansion of quartz (approx. 5.5.times.10.sup.-7) is lower than that of silicon (3.times.10.sup.-6 .times.C.sup.-1) by a factor of 10. Since silicon unites firmly with the walls of the quartz chill mold during solidification, tensions are built up during the solidification and cooling of the silicon block, resulting in cracks in the silicon block and in breakage of the quartz chill mold. In addition, quartz vessels tend to devitrify and shatter on cooling from the necessary temperature range. Accordingly, they cannot be reused and, by virtue of their high cost, contribute considerably towards increasing the overall cost of the silicon crystallization process.
In order to avoid direct contact between the silicon melt and the quartz wall and, hence, fusion of the silicon block and quartz crucible, quartz crucibles coated with silicon nitride have been proposed [T. Saito, A. Shimura, S. Ishikawa, Solar Energy Materials 9, 337-345 (1983)].
Owing to devitrification, however, materials thus coated are also reusable to only a limited extent. The use of quartz moldings is also limited by increasing softening of the material with increasing temperature of the silicon melt, so that there is a practical limit at around 1500.degree. C. for the use of quartz for handling silicon melts.
Higher temperatures can be reached with graphite. Since, in addition, graphite is very easy to machine, its use for handling metal melts appears to be very attractive. Graphite qualities of high density, clearly pronounced crystallinity and relatively little open porosity are commercially obtainable and show adequate chemical resistance to silicon melts.
On contact with liquid silicon, graphite forms a thin SiC layer. Through the diffusion of carbon into the melt, secretions of SiC are formed when the solubility limit is exceeded. It is a well known phenomenon that SiC secretions significantly reduce the efficiency of solar cells.
Compact-sintered silicon nitride moldings can be produced by hot pressing of Si.sub.3 N.sub.4 using sinter additives. However, the processes involved are complicated and expensive so that moldings produced in this way cannot contribute towards reducing costs in the handling of silicon melts.
A general disadvantage of Si.sub.3 N.sub.4 ceramics is that the moldings can only be aftertreated with diamond-tipped tools which is both difficult and expensive.
In order therefore effectively to reduce the costs involved in a process for handling silicon, particularly in fusion and crystallization processes, the problem arose of replacing the expensive materials hitherto used, namely quartz, carbon and Si.sub.3 N.sub.4 ceramics with their above-described disadvantages, by a new, inexpensive and easy-to-produce material which does not have these disadvantages.
These requirements are excellently satisfied by the material according to the invention described hereinafter.